1. Field of the Invention
This device relates to the art of ophthalmic lens grinding. In particular, an apparatus is described for blocking a lens by vacuum. The apparatus includes a feature which will maintain the vacuum without the necessity of vacuum hoses.
2. Description of the Prior Art
One of the most troublesome aspects in finishing lens blanks to prescription curvatures is the gripping of the blank during processing. The lens blank itself is either glass or plastic, it is somewhat fragile, and must be securely gripped in a manner to withstand grinding stresses. The prior art teaches many ingenious ways of "blocking" a lens for this purpose. A common method is to secure a steel block to one lens surface through the use of pitch or the like (See Fritzche U.S. Pat. No. 2,465,153). Another common method is to employ a low melting point alloy to attach the lens to the block (See U.S. Pat. No. 3,468,366, for instance). In each of the prior art methods, the lens blank is oriented with respect to the block. After finishing, the block must be removed and the lens cleaned. The alloy must be recovered for economic reasons and the pitch is a troublesome product with which to work.
A new method has been recently developed for blocking lens blanks which includes the use of an evacuated cavity to hold the lens to the block. Such an application is described in U.S. Pat. No. 3,794,314 issued Feb. 26, 1974. The technique disclosed therein is to place a lens on a block having a sealed cavity where the lens rests and then withdrawing the air from the cavity so that vacuum holds the lens in place on the block. The vacuum in the cavity is maintained by means of a pneumatic hose connected to a vacuum drawing pump. The present invention provides a system wherein a lens can be blocked by vacuum and that vacuum mantained without the encumbering vacuum hose.